Wind turbines are being designed ever larger due to the economic and political incentives to increase energy production from renewable resources.
As the overall size of wind turbines increase, so too do the forces experienced by the wind turbines in operation. One significant factor in tower loading is the force generated due to the motion of the rotor mounted to the nacelle of the wind turbine. In ideal circumstances, the rotor would be balanced so as to minimise the forces applied to the tower by this source of excitation. However, in practice, the rotor generates cyclical forces on the tower due to two principle causes: aerodynamic imbalance and mass imbalance. Aerodynamic imbalance can occur when the aerodynamic properties of the blades are affected, for example when one or more of the blades are mounted incorrectly, when one blade is dirtier than the others, or when ice-build up on one of the blades is more severe. Aerodynamic imbalance can also occur when turbulent regions of airflow passing through the rotor plane affect the blades unequally. Mass imbalance can occur when the mass of the blades are affected, for example if the mass of the blades are different at installation, or due to water accumulation in the interior of the blades.
The tower will oscillate in accordance with its natural frequency or ‘eigenfrequency’ which is determined largely by structural features of the wind turbine such as its height, diameter, material of fabrication, nacelle mass to name a few factors. Typically, a wind turbine will be designed such that the eigenfrequency of the tower is spaced, in the frequency domain, from the operational speed range of the rotor and associated generating equipment. However, this design principle means that the influence of rotor imbalance on the tower is difficult to detect and quantify, the result being that important components of the system, such as the rotor bearings, generating equipment and the like are subject to unbalanced forces that can have a detrimental impact on their service life.
Some efforts have been made to diagnose rotor imbalance for wind turbines. In one study, as documented in “Caselitz, P., Giebhardt, J.: Rotor Condition Monitoring for Improved Operational Safety of Offshore Wind Energy Converters. ASME Journal of Solar Energy Engineering 2005, 127, p253-261”, a statistical approach is taken to diagnose a mass imbalance between the blades of a wind turbine. In particular, this approach applies a ‘learning phase’ over a significant time period (presented as three months) during which the system monitors the power output and wind speed conditions in order to define a power characteristic for a ‘faultless’ rotor. Further measurements are then taken to identify any departure from the ‘faultless’ characteristic in order to identify that a problem exits with the rotor. Instrumentation in the form of nacelle-mounted accelerometers then provide data which is analysed to determine if a mass imbalance exists between the blades of the rotor. Although such a scheme appears to provide an approach which offers the potential to diagnose blade imbalance conditions, in practice it is impractical due to the need for the learning phase to characterise a ‘faultless’ rotor, and due to its reliance on the assumption that the rotor as installed will indeed be faultless. A further challenge is how to separate out the effects of mass imbalance and pitch imbalance on the statistical data.
It is against this background that the invention has been devised.